Systems whose proper operation requires temperature regulation typically employ some form of electric thermostat to control a heating or cooling source to maintain the temperature at a desired set point or within a desired range. Such systems are widely varied, and include deep fryers, griddles, ovens, boilers, etc. These high temperature applications typically use an electric thermostat that utilizes a bulb and capillary tube to actuate a control mechanism at a user selected, or preset temperature.
In one exemplary system, to wit a deep fryer in a commercial restaurant application, a gas burner is utilized to provide the thermal energy to the oil bath used for deep frying. The gas flowing to the burner is controlled by a gas flow control valve. The positioning of the valve, for example open or closed, is regulated by an electric thermostat whose temperature sensing bulb is positioned within the oil bath used for deep frying. To eliminate the need for an external power source, the gas flow control valve may utilize a thermopile wire as is well known in the art. The electric thermostat typically includes a knob or other user interface to allow the user to set the desired temperature for the oil bath.
During operation, the thermostat monitors the temperature of the oil bath via the immersed bulb. If the monitored temperature of the oil bath is below the set point temperature of the thermostat, the gas flow control valve is opened to allow the flow of gas to the burner. The burner ignition system then ignites the gas at the burner to raise the temperature of the oil bath. Once the temperature reaches the set point of the thermostat, the thermostat mechanism switches off the gas flow control valve to stop the flow of gas to the burner. As the temperature of the oil bath begins to drop, the automatic reset function of the electric thermostat switches to again command the gas flow control valve to open to once again allow the flow of gas to the burner. In this way, the oil in the oil bath is maintained at the desired set point temperature, within a range, to ensure proper cooking of the deep fried foods.
An electric thermostat particularly well suited to high temperature applications such as that described above is the model RX Millivolt Direct Current Electric Thermostat sold by Robertshaw Controls Company. This single pole thermostat is designed especially for demanding millivolt/milliamp direct current applications, such as those that utilize a thermopile to eliminate the need for an external power source. This Model RX electric thermostat includes a hermetically sealed reed switch to provide durability and accuracy in the harshest environments with ambient temperatures reaching 230° F. This thermostat includes a rugged steel case design with screw type terminals to ensure electrical integrity in such harsh environments, and provides a precise and proven snap action mechanism to control, for example, a gas flow control valve.
Operation of this proven model RX electric thermostat may be better understood with reference to the cross-sectional illustration of this thermostat of FIG. 14. This cross-sectional illustration of the model RX electric thermostat 21 illustrates the positioning of the snap-action mechanism 23 such that the contacts of the reed switch 25 are open circuit. The reed switch 25 is carried in a terminal block 27 that serves as the carrier for the electrical terminal connections 29. Within the terminal block 27, the actuating mechanism 31 of the snap action mechanism 23 is allowed to transition between a first position illustrated in this FIG. 14 and a second illustrated in FIG. 15. The transitioning of the actuating mechanism 31 between these two positions causes the contacts of the reed switch 25 to transition between open and closed positions under the influence of magnet 33 carried by the actuating mechanism 31. That is, as the magnet 33 is moved away from the reed switch 25 (see FIG. 14), the contacts of the reed switch 25 open. However, when the magnet 33 is brought into close proximity to the reed switch 25 (FIG. 15), the contacts of the reed switch 25 close.
The transitioning between the two positions of the actuating mechanism 31 is accomplished when the fluid in the temperature sensing bulb expands and contracts with the temperature variation in the media sensed by the bulb. As the fluid expands, it causes a deformation in the diastat 35 which causes the actuator post 37 to push down on an actuating dimple 39 of the snap action mechanism 23. Once a sufficient deformation of the diastat 35 has caused a sufficient lateral translation of the actuating post 37, the snap action mechanism operates to cause a rapid position change of the actuating mechanism 31. Similarly, as the temperature in the sensed media drops, the fluid in the temperature sensing bulb contracts. As the fluid is evacuated down the capillary tube, the diastat 35 is allowed to return to its undeformed position, thereby effectuating a lateral translation of the actuating post 37 away from the dimple 39. Once a sufficient translation has occurred, the snap action mechanism 23 again functions to provide a rapid transition of the actuating mechanism 31 to its alternate position. To aid in this precise movement between the two positions of the actuating mechanism 31, and to minimize the amount of bounce that may occur in the actuating mechanism during this transition, a pair of positioning springs 41, 43 are carried in the actuating mechanism 31 within the terminal block 27. As discussed above, a user actuated knob may be provided on the adjusting screw 45 to allow the user to vary the operating point of the snap action mechanism 23.
In many of the applications discussed above, a failure of the thermostat to turn off the gas flow control valve may result in overheating of the oil bath, oven cavity, griddle surface, etc., as the burner continues to supply thermal energy. While most systems provide a manual shutoff of the burner, such requires that a person sense the overheating condition and turn off the burner. If this condition is not noticed, however, the thermal overheating may result in food being overcooked or the existence of a hazardous condition.
There exists a need in the art, therefore, for a high temperature limit backup thermostat that will shut off the burner upon failure of the primary control thermostat prior to reaching a hazardous temperature.